1 |
NAME |
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AnyEvent::Fork::RPC - simple RPC extension for AnyEvent::Fork |
3 |
|
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THE API IS NOT FINISHED, CONSIDER THIS A BETA RELEASE |
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|
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SYNOPSIS |
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use AnyEvent::Fork::RPC; |
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# use AnyEvent::Fork is not needed |
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|
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my $rpc = AnyEvent::Fork |
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->new |
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->require ("MyModule") |
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->AnyEvent::Fork::RPC::run ( |
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"MyModule::server", |
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); |
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|
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use AnyEvent; |
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|
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my $cv = AE::cv; |
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|
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$rpc->(1, 2, 3, sub { |
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print "MyModule::server returned @_\n"; |
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$cv->send; |
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}); |
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|
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$cv->recv; |
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|
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DESCRIPTION |
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This module implements a simple RPC protocol and backend for processes |
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created via AnyEvent::Fork, allowing you to call a function in the child |
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process and receive its return values (up to 4GB serialised). |
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|
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It implements two different backends: a synchronous one that works like |
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a normal function call, and an asynchronous one that can run multiple |
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jobs concurrently in the child, using AnyEvent. |
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|
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It also implements an asynchronous event mechanism from the child to the |
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parent, that could be used for progress indications or other |
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information. |
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|
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Loading this module also always loads AnyEvent::Fork, so you can make a |
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separate "use AnyEvent::Fork" if you wish, but you don't have to. |
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|
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EXAMPLES |
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Example 1: Synchronous Backend |
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Here is a simple example that implements a backend that executes |
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"unlink" and "rmdir" calls, and reports their status back. It also |
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reports the number of requests it has processed every three requests, |
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which is clearly silly, but illustrates the use of events. |
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|
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First the parent process: |
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|
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use AnyEvent; |
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use AnyEvent::Fork::RPC; |
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|
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my $done = AE::cv; |
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|
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my $rpc = AnyEvent::Fork |
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->new |
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->require ("MyWorker") |
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->AnyEvent::Fork::RPC::run ("MyWorker::run", |
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on_error => sub { warn "FATAL: $_[0]"; exit 1 }, |
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on_event => sub { warn "$_[0] requests handled\n" }, |
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on_destroy => $done, |
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); |
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|
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for my $id (1..6) { |
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$rpc->(rmdir => "/tmp/somepath/$id", sub { |
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$_[0] |
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or warn "/tmp/somepath/$id: $_[1]\n"; |
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}); |
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} |
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|
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undef $rpc; |
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|
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$done->recv; |
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|
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The parent creates the process, queues a few rmdir's. It then forgets |
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about the $rpc object, so that the child exits after it has handled the |
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requests, and then it waits till the requests have been handled. |
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|
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The child is implemented using a separate module, "MyWorker", shown |
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here: |
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|
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package MyWorker; |
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|
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my $count; |
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|
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sub run { |
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my ($cmd, $path) = @_; |
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|
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AnyEvent::Fork::RPC::event ($count) |
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unless ++$count % 3; |
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|
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my $status = $cmd eq "rmdir" ? rmdir $path |
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: $cmd eq "unlink" ? unlink $path |
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: die "fatal error, illegal command '$cmd'"; |
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|
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$status or (0, "$!") |
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} |
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|
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1 |
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|
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The "run" function first sends a "progress" event every three calls, and |
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then executes "rmdir" or "unlink", depending on the first parameter (or |
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dies with a fatal error - obviously, you must never let this happen :). |
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|
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Eventually it returns the status value true if the command was |
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successful, or the status value 0 and the stringified error message. |
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|
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On my system, running the first code fragment with the given MyWorker.pm |
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in the current directory yields: |
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|
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/tmp/somepath/1: No such file or directory |
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/tmp/somepath/2: No such file or directory |
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3 requests handled |
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/tmp/somepath/3: No such file or directory |
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/tmp/somepath/4: No such file or directory |
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/tmp/somepath/5: No such file or directory |
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6 requests handled |
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/tmp/somepath/6: No such file or directory |
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|
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Obviously, none of the directories I am trying to delete even exist. |
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Also, the events and responses are processed in exactly the same order |
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as they were created in the child, which is true for both synchronous |
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and asynchronous backends. |
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|
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Note that the parentheses in the call to "AnyEvent::Fork::RPC::event" |
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are not optional. That is because the function isn't defined when the |
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code is compiled. You can make sure it is visible by pre-loading the |
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correct backend module in the call to "require": |
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|
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->require ("AnyEvent::Fork::RPC::Sync", "MyWorker") |
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|
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Since the backend module declares the "event" function, loading it first |
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ensures that perl will correctly interpret calls to it. |
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|
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And as a final remark, there is a fine module on CPAN that can |
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asynchronously "rmdir" and "unlink" and a lot more, and more efficiently |
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than this example, namely IO::AIO. |
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|
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Example 1a: the same with the asynchronous backend |
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This example only shows what needs to be changed to use the async |
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backend instead. Doing this is not very useful, the purpose of this |
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example is to show the minimum amount of change that is required to go |
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from the synchronous to the asynchronous backend. |
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|
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To use the async backend in the previous example, you need to add the |
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"async" parameter to the "AnyEvent::Fork::RPC::run" call: |
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|
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->AnyEvent::Fork::RPC::run ("MyWorker::run", |
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async => 1, |
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... |
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|
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And since the function call protocol is now changed, you need to adopt |
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"MyWorker::run" to the async API. |
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|
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First, you need to accept the extra initial $done callback: |
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|
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sub run { |
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my ($done, $cmd, $path) = @_; |
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|
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And since a response is now generated when $done is called, as opposed |
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to when the function returns, we need to call the $done function with |
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the status: |
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|
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$done->($status or (0, "$!")); |
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|
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A few remarks are in order. First, it's quite pointless to use the async |
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backend for this example - but it *is* possible. Second, you can call |
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$done before or after returning from the function. Third, having both |
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returned from the function and having called the $done callback, the |
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child process may exit at any time, so you should call $done only when |
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you really *are* done. |
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|
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Example 2: Asynchronous Backend |
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This example implements multiple count-downs in the child, using |
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AnyEvent timers. While this is a bit silly (one could use timers in te |
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parent just as well), it illustrates the ability to use AnyEvent in the |
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child and the fact that responses can arrive in a different order then |
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the requests. |
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|
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It also shows how to embed the actual child code into a "__DATA__" |
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section, so it doesn't need any external files at all. |
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|
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And when your parent process is often busy, and you have stricter timing |
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requirements, then running timers in a child process suddenly doesn't |
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look so silly anymore. |
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|
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Without further ado, here is the code: |
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|
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use AnyEvent; |
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use AnyEvent::Fork::RPC; |
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|
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my $done = AE::cv; |
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|
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my $rpc = AnyEvent::Fork |
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->new |
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->require ("AnyEvent::Fork::RPC::Async") |
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->eval (do { local $/; <DATA> }) |
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->AnyEvent::Fork::RPC::run ("run", |
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async => 1, |
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on_error => sub { warn "FATAL: $_[0]"; exit 1 }, |
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on_event => sub { print $_[0] }, |
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on_destroy => $done, |
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); |
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|
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for my $count (3, 2, 1) { |
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$rpc->($count, sub { |
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warn "job $count finished\n"; |
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}); |
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} |
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|
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undef $rpc; |
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|
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$done->recv; |
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|
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__DATA__ |
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|
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# this ends up in main, as we don't use a package declaration |
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|
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use AnyEvent; |
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|
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sub run { |
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my ($done, $count) = @_; |
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|
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my $n; |
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|
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AnyEvent::Fork::RPC::event "starting to count up to $count\n"; |
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|
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my $w; $w = AE::timer 1, 1, sub { |
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++$n; |
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|
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AnyEvent::Fork::RPC::event "count $n of $count\n"; |
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|
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if ($n == $count) { |
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undef $w; |
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$done->(); |
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} |
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}; |
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} |
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|
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The parent part (the one before the "__DATA__" section) isn't very |
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different from the earlier examples. It sets async mode, preloads the |
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backend module (so the "AnyEvent::Fork::RPC::event" function is |
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declared), uses a slightly different "on_event" handler (which we use |
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simply for logging purposes) and then, instead of loading a module with |
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the actual worker code, it "eval"'s the code from the data section in |
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the child process. |
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|
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It then starts three countdowns, from 3 to 1 seconds downwards, destroys |
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the rpc object so the example finishes eventually, and then just waits |
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for the stuff to trickle in. |
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|
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The worker code uses the event function to log some progress messages, |
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but mostly just creates a recurring one-second timer. |
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|
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The timer callback increments a counter, logs a message, and eventually, |
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when the count has been reached, calls the finish callback. |
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|
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On my system, this results in the following output. Since all timers |
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fire at roughly the same time, the actual order isn't guaranteed, but |
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the order shown is very likely what you would get, too. |
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|
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starting to count up to 3 |
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starting to count up to 2 |
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starting to count up to 1 |
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count 1 of 3 |
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count 1 of 2 |
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count 1 of 1 |
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job 1 finished |
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count 2 of 2 |
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job 2 finished |
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count 2 of 3 |
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count 3 of 3 |
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job 3 finished |
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|
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While the overall ordering isn't guaranteed, the async backend still |
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guarantees that events and responses are delivered to the parent process |
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in the exact same ordering as they were generated in the child process. |
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|
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And unless your system is *very* busy, it should clearly show that the |
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job started last will finish first, as it has the lowest count. |
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|
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This concludes the async example. Since AnyEvent::Fork does not actually |
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fork, you are free to use about any module in the child, not just |
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AnyEvent, but also IO::AIO, or Tk for example. |
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|
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PARENT PROCESS USAGE |
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This module exports nothing, and only implements a single function: |
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|
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my $rpc = AnyEvent::Fork::RPC::run $fork, $function, [key => value...] |
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The traditional way to call it. But it is way cooler to call it in |
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the following way: |
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|
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my $rpc = $fork->AnyEvent::Fork::RPC::run ($function, [key => value...]) |
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This "run" function/method can be used in place of the |
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AnyEvent::Fork::run method. Just like that method, it takes over the |
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AnyEvent::Fork process, but instead of calling the specified |
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$function directly, it runs a server that accepts RPC calls and |
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handles responses. |
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|
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It returns a function reference that can be used to call the |
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function in the child process, handling serialisation and data |
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transfers. |
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|
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The following key/value pairs are allowed. It is recommended to have |
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at least an "on_error" or "on_event" handler set. |
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|
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on_error => $cb->($msg) |
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Called on (fatal) errors, with a descriptive (hopefully) |
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message. If this callback is not provided, but "on_event" is, |
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then the "on_event" callback is called with the first argument |
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being the string "error", followed by the error message. |
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|
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If neither handler is provided it prints the error to STDERR and |
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will start failing badly. |
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|
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on_event => $cb->(...) |
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Called for every call to the "AnyEvent::Fork::RPC::event" |
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function in the child, with the arguments of that function |
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passed to the callback. |
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|
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Also called on errors when no "on_error" handler is provided. |
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|
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on_destroy => $cb->() |
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Called when the $rpc object has been destroyed and all requests |
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have been successfully handled. This is useful when you queue |
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some requests and want the child to go away after it has handled |
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them. The problem is that the parent must not exit either until |
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all requests have been handled, and this can be accomplished by |
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waiting for this callback. |
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|
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init => $function (default none) |
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When specified (by name), this function is called in the child |
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as the very first thing when taking over the process, with all |
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the arguments normally passed to the "AnyEvent::Fork::run" |
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function, except the communications socket. |
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|
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It can be used to do one-time things in the child such as |
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storing passed parameters or opening database connections. |
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|
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It is called very early - before the serialisers are created or |
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the $function name is resolved into a function reference, so it |
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could be used to load any modules that provide the serialiser or |
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function. It can not, however, create events. |
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|
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async => $boolean (default: 0) |
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The default server used in the child does all I/O blockingly, |
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and only allows a single RPC call to execute concurrently. |
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|
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Setting "async" to a true value switches to another |
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implementation that uses AnyEvent in the child and allows |
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multiple concurrent RPC calls (it does not support recursion in |
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the event loop however, blocking condvar calls will fail). |
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|
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The actual API in the child is documented in the section that |
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describes the calling semantics of the returned $rpc function. |
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|
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If you want to pre-load the actual back-end modules to enable |
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memory sharing, then you should load "AnyEvent::Fork::RPC::Sync" |
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for synchronous, and "AnyEvent::Fork::RPC::Async" for |
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asynchronous mode. |
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|
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If you use a template process and want to fork both sync and |
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async children, then it is permissible to load both modules. |
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|
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serialiser => $string (default: |
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$AnyEvent::Fork::RPC::STRING_SERIALISER) |
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All arguments, result data and event data have to be serialised |
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to be transferred between the processes. For this, they have to |
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be frozen and thawed in both parent and child processes. |
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|
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By default, only octet strings can be passed between the |
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processes, which is reasonably fast and efficient and requires |
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no extra modules. |
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|
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For more complicated use cases, you can provide your own freeze |
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and thaw functions, by specifying a string with perl source |
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code. It's supposed to return two code references when |
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evaluated: the first receives a list of perl values and must |
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return an octet string. The second receives the octet string and |
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must return the original list of values. |
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|
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If you need an external module for serialisation, then you can |
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either pre-load it into your AnyEvent::Fork process, or you can |
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add a "use" or "require" statement into the serialiser string. |
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Or both. |
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|
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Here are some examples - some of them are also available as |
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global variables that make them easier to use. |
392 |
|
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octet strings - $AnyEvent::Fork::RPC::STRING_SERIALISER |
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This serialiser concatenates length-prefixes octet strings, |
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and is the default. |
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|
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Implementation: |
398 |
|
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( |
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sub { pack "(w/a*)*", @_ }, |
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sub { unpack "(w/a*)*", shift } |
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) |
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|
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json - $AnyEvent::Fork::RPC::JSON_SERIALISER |
405 |
This serialiser creates JSON arrays - you have to make sure |
406 |
the JSON module is installed for this serialiser to work. It |
407 |
can be beneficial for sharing when you preload the JSON |
408 |
module in a template process. |
409 |
|
410 |
JSON (with JSON::XS installed) is slower than the octet |
411 |
string serialiser, but usually much faster than Storable, |
412 |
unless big chunks of binary data need to be transferred. |
413 |
|
414 |
Implementation: |
415 |
|
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use JSON (); |
417 |
( |
418 |
sub { JSON::encode_json \@_ }, |
419 |
sub { @{ JSON::decode_json shift } } |
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) |
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|
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storable - $AnyEvent::Fork::RPC::STORABLE_SERIALISER |
423 |
This serialiser uses Storable, which means it has high |
424 |
chance of serialising just about anything you throw at it, |
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at the cost of having very high overhead per operation. It |
426 |
also comes with perl. |
427 |
|
428 |
Implementation: |
429 |
|
430 |
use Storable (); |
431 |
( |
432 |
sub { Storable::freeze \@_ }, |
433 |
sub { @{ Storable::thaw shift } } |
434 |
) |
435 |
|
436 |
See the examples section earlier in this document for some actual |
437 |
examples. |
438 |
|
439 |
$rpc->(..., $cb->(...)) |
440 |
The RPC object returned by "AnyEvent::Fork::RPC::run" is actually a |
441 |
code reference. There are two things you can do with it: call it, |
442 |
and let it go out of scope (let it get destroyed). |
443 |
|
444 |
If "async" was false when $rpc was created (the default), then, if |
445 |
you call $rpc, the $function is invoked with all arguments passed to |
446 |
$rpc except the last one (the callback). When the function returns, |
447 |
the callback will be invoked with all the return values. |
448 |
|
449 |
If "async" was true, then the $function receives an additional |
450 |
initial argument, the result callback. In this case, returning from |
451 |
$function does nothing - the function only counts as "done" when the |
452 |
result callback is called, and any arguments passed to it are |
453 |
considered the return values. This makes it possible to "return" |
454 |
from event handlers or e.g. Coro threads. |
455 |
|
456 |
The other thing that can be done with the RPC object is to destroy |
457 |
it. In this case, the child process will execute all remaining RPC |
458 |
calls, report their results, and then exit. |
459 |
|
460 |
See the examples section earlier in this document for some actual |
461 |
examples. |
462 |
|
463 |
CHILD PROCESS USAGE |
464 |
The following function is not available in this module. They are only |
465 |
available in the namespace of this module when the child is running, |
466 |
without having to load any extra modules. They are part of the |
467 |
child-side API of AnyEvent::Fork::RPC. |
468 |
|
469 |
AnyEvent::Fork::RPC::event ... |
470 |
Send an event to the parent. Events are a bit like RPC calls made by |
471 |
the child process to the parent, except that there is no notion of |
472 |
return values. |
473 |
|
474 |
See the examples section earlier in this document for some actual |
475 |
examples. |
476 |
|
477 |
ADVANCED TOPICS |
478 |
Choosing a backend |
479 |
So how do you decide which backend to use? Well, that's your problem to |
480 |
solve, but here are some thoughts on the matter: |
481 |
|
482 |
Synchronous |
483 |
The synchronous backend does not rely on any external modules (well, |
484 |
except common::sense, which works around a bug in how perl's warning |
485 |
system works). This keeps the process very small, for example, on my |
486 |
system, an empty perl interpreter uses 1492kB RSS, which becomes |
487 |
2020kB after "use warnings; use strict" (for people who grew up with |
488 |
C64s around them this is probably shocking every single time they |
489 |
see it). The worker process in the first example in this document |
490 |
uses 1792kB. |
491 |
|
492 |
Since the calls are done synchronously, slow jobs will keep newer |
493 |
jobs from executing. |
494 |
|
495 |
The synchronous backend also has no overhead due to running an event |
496 |
loop - reading requests is therefore very efficient, while writing |
497 |
responses is less so, as every response results in a write syscall. |
498 |
|
499 |
If the parent process is busy and a bit slow reading responses, the |
500 |
child waits instead of processing further requests. This also limits |
501 |
the amount of memory needed for buffering, as never more than one |
502 |
response has to be buffered. |
503 |
|
504 |
The API in the child is simple - you just have to define a function |
505 |
that does something and returns something. |
506 |
|
507 |
It's hard to use modules or code that relies on an event loop, as |
508 |
the child cannot execute anything while it waits for more input. |
509 |
|
510 |
Asynchronous |
511 |
The asynchronous backend relies on AnyEvent, which tries to be |
512 |
small, but still comes at a price: On my system, the worker from |
513 |
example 1a uses 3420kB RSS (for AnyEvent, which loads EV, which |
514 |
needs XSLoader which in turn loads a lot of other modules such as |
515 |
warnings, strict, vars, Exporter...). |
516 |
|
517 |
It batches requests and responses reasonably efficiently, doing only |
518 |
as few reads and writes as needed, but needs to poll for events via |
519 |
the event loop. |
520 |
|
521 |
Responses are queued when the parent process is busy. This means the |
522 |
child can continue to execute any queued requests. It also means |
523 |
that a child might queue a lot of responses in memory when it |
524 |
generates them and the parent process is slow accepting them. |
525 |
|
526 |
The API is not a straightforward RPC pattern - you have to call a |
527 |
"done" callback to pass return values and signal completion. Also, |
528 |
more importantly, the API starts jobs as fast as possible - when |
529 |
1000 jobs are queued and the jobs are slow, they will all run |
530 |
concurrently. The child must implement some queueing/limiting |
531 |
mechanism if this causes problems. Alternatively, the parent could |
532 |
limit the amount of rpc calls that are outstanding. |
533 |
|
534 |
Blocking use of condvars is not supported. |
535 |
|
536 |
Using event-based modules such as IO::AIO, Gtk2, Tk and so on is |
537 |
easy. |
538 |
|
539 |
Passing file descriptors |
540 |
Unlike AnyEvent::Fork, this module has no in-built file handle or file |
541 |
descriptor passing abilities. |
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|
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The reason is that passing file descriptors is extraordinary tricky |
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business, and conflicts with efficient batching of messages. |
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|
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There still is a method you can use: Create a |
547 |
"AnyEvent::Util::portable_socketpair" and "send_fh" one half of it to |
548 |
the process before you pass control to "AnyEvent::Fork::RPC::run". |
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|
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Whenever you want to pass a file descriptor, send an rpc request to the |
551 |
child process (so it expects the descriptor), then send it over the |
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other half of the socketpair. The child should fetch the descriptor from |
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the half it has passed earlier. |
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|
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Here is some (untested) pseudocode to that effect: |
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|
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use AnyEvent::Util; |
558 |
use AnyEvent::Fork::RPC; |
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use IO::FDPass; |
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|
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my ($s1, $s2) = AnyEvent::Util::portable_socketpair; |
562 |
|
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my $rpc = AnyEvent::Fork |
564 |
->new |
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->send_fh ($s2) |
566 |
->require ("MyWorker") |
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->AnyEvent::Fork::RPC::run ("MyWorker::run" |
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init => "MyWorker::init", |
569 |
); |
570 |
|
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undef $s2; # no need to keep it around |
572 |
|
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# pass an fd |
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$rpc->("i'll send some fd now, please expect it!", my $cv = AE::cv); |
575 |
|
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IO::FDPass fileno $s1, fileno $handle_to_pass; |
577 |
|
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$cv->recv; |
579 |
|
580 |
The MyWorker module could look like this: |
581 |
|
582 |
package MyWorker; |
583 |
|
584 |
use IO::FDPass; |
585 |
|
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my $s2; |
587 |
|
588 |
sub init { |
589 |
$s2 = $_[0]; |
590 |
} |
591 |
|
592 |
sub run { |
593 |
if ($_[0] eq "i'll send some fd now, please expect it!") { |
594 |
my $fd = IO::FDPass::recv fileno $s2; |
595 |
... |
596 |
} |
597 |
} |
598 |
|
599 |
Of course, this might be blocking if you pass a lot of file descriptors, |
600 |
so you might want to look into AnyEvent::FDpasser which can handle the |
601 |
gory details. |
602 |
|
603 |
EXCEPTIONS |
604 |
There are no provisions whatsoever for catching exceptions at this time |
605 |
- in the child, exeptions might kill the process, causing calls to be |
606 |
lost and the parent encountering a fatal error. In the parent, |
607 |
exceptions in the result callback will not be caught and cause undefined |
608 |
behaviour. |
609 |
|
610 |
SEE ALSO |
611 |
AnyEvent::Fork, to create the processes in the first place. |
612 |
|
613 |
AnyEvent::Fork::Pool, to manage whole pools of processes. |
614 |
|
615 |
AUTHOR AND CONTACT INFORMATION |
616 |
Marc Lehmann <schmorp@schmorp.de> |
617 |
http://software.schmorp.de/pkg/AnyEvent-Fork-RPC |
618 |
|